In hydraulic brake boosters of a type disclosed in U.S. Pat. Nos. 4,281,585; 4,539,892; 4,625,515; 6,561,596 and U.S. application Ser. No. 10/307,791 filed Dec. 2, 2002, a control valve is located in a first bore and a power piston is located in a second bore of a housing. A lever arrangement is connected to the power piston and the control valve. An input force applied to a brake pedal by an operator acts on the lever arrangement to develop a manual mode and a power assist mode of operation. The lever arrangement pivots on the power piston and communicates an actuation force that moves the control valve to regulate the flow of pressurize fluid from a source to an operational chamber. The regulated pressurized fluid supplied to the operational chamber acts on the power piston to develop an operational force that pressurizes fluid in the first bore that is supplied to wheel brakes to effect a corresponding brake application. A reaction force produced by regulated pressurized fluid in the movement of the power piston is transmitted back to the brake pedal to balance the input force such that the operational force supplied to move the power piston in the first bore is a linear function of the input force applied to the brake pedal.
Additional features such as traction control, dynamic operational control and anti-skid control under the control of an ECU have been added to hydraulic brake booster to provide a total brake system. Unfortunately in some brake systems, during an ECU generated brake application, the brake pedal of the hydraulic booster mirrored the movement of the power piston. One way to hold a brake pedal stationary during an ECU generated brake application is disclosed by the structure in U.S. Pat. No. 6,203,119 wherein a control valve seat moves to meter pressurize fluid while in U.S. patent application Ser. No. 10/307,791 a separate piston is provided to act on the control valve to meter pressurized fluid to effect a brake application.